This multicenter, double-blind, placebo-controlled, randomized, parallel-group trial will investigate the safety & efficacy of rufinamide administered as adjunctive therapy in patients w/inadequately controlled primary generalized tonic-clonic (PGTC) seizures. The trial consistes of 3 phases: a 56-day baseline ,a 140-day double-blind treatment phase and a long term extension phase. Approx. 144 patients will be recruited for the study nationally, with about six patients to be enrolled at USC.